A modulator, such as a polar modulator, in a wireless communications device may require spectral quality for modulation schemes. Typically, most wireless communications devices are based on constant-envelope modulation schemes (i.e., phase or frequency modulated). An advantage with constant-envelope modulation scheme provides that a final radio frequency (RF) power amplifier in the polar modulator does not have to be linear, and as a consequence, the final RF power amplifier can be operated in the most power efficient region near saturation. However, a drawback with constant-envelope modulation scheme is the inefficient use of the RF spectrum, where data rate transmission for a given bandwidth is not maximized.
To utilize the RF spectrum efficiently, a varying envelope and varying phase modulation scheme may be used. When a varying envelope modulation is applied to a power efficient nonlinear amplifier, distortion may be generated by the nonlinear amplifier which may cause interference with adjacent channels. The distortion may also result in detection error of the information signal at the receiver end of the communication channel. For most applications, the distortion is to be avoided, and may require a linear amplifier; however, linear amplifiers typically have low power efficiency, making a linear amplifier unsuitable for varying envelope and phase modulation scheme. To obtain linear amplification and high power efficiency for transmission in the polar modulator, linearization of a power efficient and nonlinear amplifier may be implemented.
Linearization may refer to a method of compensation or correction of non-linearity in a polar modulator component to maintain stability at the output of the polar modulator. Linearization of the polar modulator may require a feedback receiver component to couple the polar modulator output into a signal path of the amplifier's input. The feedback receiver component may produce linear amplification and power efficiency in the polar modulator.
The feedback receiver component may be used as a quadrature demodulator and require additional circuitry (e.g., high frequency local oscillator, mixers, 90 degree shifters, etc.). Such additional circuitry may draw significant amounts of current in the polar modulator. Furthermore, delay sensitivity may further be included due to different delays in signal sources and local oscillator input used in the quadrature demodulator. The delay sensitivity may result in degradation of the circuitry at microwave frequencies due to sub nanosecond delay variation requirements.